Methods and Operating Procedures for Checking the Condition of a Frequency Converter with a Multimeter
As is well known, there are many protection functions, such as overcurrent, overvoltage, overload protection, and so on. With the continuous improvement of industrial automation, frequency converters have also been widely used.
In the process of circuit design, engineers inevitably need to measure some measuring instruments. Engineers know that a multimeter can measure DC current, AC voltage, and DC voltage. And the frequency converter is a device that controls AC by modifying the operating frequency of the motor. This article will explain how to use a multimeter to measure the quality of a frequency converter.
It should be noted that for personal safety, the machine must be powered off and the input power lines R, S, T and output lines U, V, W of the frequency converter must be removed before operation! First, set the multimeter to the "second level tube" position, and then use the red and black probes of the multimeter to test according to the following steps:
The black probe contacts the negative pole P (+) of the DC bus, and the red probe contacts R, S, and T in sequence, recording the displayed value on the multimeter. Then touch the red probe to N (-), and touch the black probe to R, S, and T in sequence, recording the display value of the multimeter. If the six displayed values are basically balanced, it indicates that there is no problem with the rectifier or soft start resistor of the frequency converter. Otherwise, if the rectifier module or soft start resistor at the corresponding position is damaged, the phenomenon is: no display.
The red probe contacts the negative pole P (+) of the DC bus, and the black probe contacts U, V, and W in sequence, recording the displayed value on the multimeter. Then touch the black probe to N (-) and the red probe to U, V, and W in sequence, and record the display value of the multimeter. If the six displayed values are basically balanced, it indicates that there is no problem with the IGBT inverter module of the frequency converter. Otherwise, if the IGBT inverter module at the corresponding position is damaged, the phenomenon is: no output or a fault is reported.
Use a frequency converter to drive an asynchronous motor with matching power on site for no-load operation, adjust the frequency f from 50Hz to the lowest frequency.
During this process, use an ammeter to detect the no-load current of the motor. If the no-load current remains stable during the frequency decrease and can remain basically unchanged, then it is a good frequency converter.
The minimum frequency can be calculated as follows: (synchronous speed - rated speed) x pole pairs p ÷ 60. For example, a 4-pole motor with a rated speed of 1470 revolutions per minute and a minimum frequency of (1500-1470) × 2 ÷ 60=1Hz.
There is no problem with the soft start resistor, otherwise the rectifier module or soft start resistor at the corresponding position is damaged, and there is no display.
The red probe contacts the negative pole P (+) of the DC bus, and the black probe contacts U, V, and W in sequence, recording the displayed value on the multimeter. Then touch the black probe to N (-) and the red probe to U, V, and W in sequence, and record the display value of the multimeter. If the six displayed values are basically balanced, it indicates that there is no problem with the IGBT inverter module of the frequency converter. Otherwise, if the IGBT inverter module at the corresponding position is damaged, the phenomenon is: no output or a fault is reported.
Use a frequency converter to drive an asynchronous motor with matching power on site for no-load operation, adjust the frequency f from 50Hz to the lowest frequency.
During this process, use an ammeter to detect the no-load current of the motor. If the no-load current remains stable during the frequency decrease and can remain basically unchanged, then it is a good frequency converter.
The minimum frequency can be calculated as follows: (synchronous speed - rated speed) x pole pairs p ÷ 60. For example, a 4-pole motor with a rated speed of 1470 revolutions per minute and a minimum frequency of (1500-1470) × 2 ÷ 60=1Hz.
Identification of AC and DC Solid State: Usually, the input and output terminals of the DC solid-state relay housing are marked with "+" and "-" symbols, and labeled with the words "Dc input" and "DC output". However, communication solid-state relays can only be marked with "+" and "-" symbols on the input end, and there is no distinction between positive and negative on the output end.
Discrimination between input and output terminals: For unmarked solid-state relays, the R × 10k range of a multimeter is used to distinguish between input and output terminals by measuring the forward and reverse resistance values of each pin separately. When the forward resistance of two pins is small and the reverse resistance is infinite, these two pins are the input terminals, and the other two pins are the output terminals. In a measurement with a smaller resistance value, the black probe is connected to the positive input terminal and the red probe is connected to the negative input terminal.
If the forward and reverse resistance of two pins are both zero, it indicates that the solid-state relay has been broken down and damaged. If the forward and reverse resistance values of each pin of the solid-state relay are measured to be infinite, it indicates that the solid-state relay has been opened and damaged.
